1. Field of the Invention
The present invention relates to wrapping a load with packaging material, and more particularly, to stabilizing the load while the load is wrapped with packaging material.
2. Description of the Related Art
Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, and protection and waterproofing. One system uses stretch wrapping machines to stretch, dispense, and wrap stretch wrap packaging material around a load. Stretch wrapping can be performed as an inline automated packaging technique which dispenses and wraps packaging material in a stretched condition around a load on a pallet to cover and contain the load. Pallet stretch wrapping, whether accomplished by turntable, rotating arm, or rotating ring typically covers the four vertical sides of the load with a stretchable film such as polyethylene film. In each of these arrangements, relative rotation is provided between the load and a packaging material dispenser,to wrap packaging material about the sides of the load.
Wrapping packaging material about the sides of the load typically unitizes and stabilizes the load. However, such side wrapping by itself generally does not secure the load to the pallet in a manner which would promote increased stability. Due to the structure of a typical stretch wrap apparatus, it is difficult to stabilize the load during wrapping and to secure the load to the pallet in a stable manner.
To increase stability to the load during the wrapping cycle, top platens are used. FIG. 2 illustrates how a conventional top platen 101 typically provides stability to a load 102 being wrapped. In FIG. 2, a load 102 is “column stacked.” In a column stacked configuration, boxes 110 (or whatever is stacked to comprise the load 102) are not interlocked with layers above or below, rather, box walls are flush. This is the most unstable type of load. Without a top platen 101 or other type of support, rotation often leads to column tilt 124, shown in phantom in FIG. 2, and sometimes to collapse. The top platen 101 provides stability in at least two ways. First, a vertical compressive force 104 is applied to the load 102 and to the turntable plate, forcing the faces of the boxes to square themselves vertically 123. Second, the applied force 104 makes use of friction between a platen pad 105 and the load 102 to create a restrictive force 125, which keeps the boxes 110 from sliding away from a center of rotation 140 of the load 102.
There are at least two problems common to all wrapping devices which use a conventional top platen. The first, illustrated in FIG. 4A, is that the top platen arm frame 111, shaft 112, or platen mast or frame 113 can be manufactured out of square or can flex, causing the rotational axis 149 of the platen pad 105 to be out of square and thus misaligned or unaligned with the center of rotation 140 of the load 102. The second and more serious problem, illustrated in FIG. 4B, is that the rotational axis 149 of the top platen pad 105 is fixed, passing through the geometric center or longitudinal axis of the shaft and platen pad of the top platen. Almost always, this means that the rotational axis 149 of the platen pad 105 is in a different location than the center of rotation 140 of the top of the load 102. The top platen pad 105 can be adjusted so its rotational axis 149 will closely match the load center of rotation 140 at some vertical position. But as shown in FIG. 10, the center of rotation 140 of the top of load 102 may be at a different position with respect to the rotational axis 149 of the top platen 105 for loads of different heights as seen at points A, B, and C. Wear or tolerance in the manufacturing of the turntable 122 may result in an uneven turntable surface, thus affecting the location of the center of rotation of the turntable and ultimately the center of rotation 140 of the load 102. Thus, when the vertical position of the top of the load changes, i.e., a short load is wrapped after a tall load is wrapped, the rotational axis 149 of the platen pad 105 may not be aligned with the center of rotation 140 of the top of the load 102.
These problems exist with all conventional top platens, regardless of whether the load or the wrapping device rotates. The problem involving the center of rotation 140 of the top of the load 102 and the rotational axis 149 of the platen pad 105 can create several undesirable effects. One of the more serious effects is that as the top of the load rotates about a different center 140 than the platen pad's rotational axis 149, the platen pad 105 and the load 102 will grind against each other. This grinding may cause wrapping material 103 placed between the load 102 and the platen pad 105 to rip, tear, or be ground away. Additionally, the load 102 itself may be affected. Loads comprised of tightly packed together units such as bricks or boxes 110, for example, may shift and move as a result of the grinding. The shifted units 110 may create open spaces in what was a tightly packed load. If the rotational axis 149 of the platen pad 105 and the rotational center of the load 140 are sufficiently misaligned, units 110 may shift and fall off of the load 102. If the rotational axis 149 of the platen pad 105 is not aligned with the center of rotation 140 of the top of the load 102, the platen device 101 may make the load 102 more unstable instead of providing stability to the load 102 due to grinding, wobbling, or pushing the load 102 out during rotation caused by misalignment.
As shown in FIG. 10, in conventional stretch wrapping devices with turntables 122, when the rotational axis 149 of the platen pad 105 and the center of rotation 140 of the turntable 122 are out of alignment, the amount of misalignment between the rotational axis 149 of the platen pad 105 and the center of rotation 140 of the top of the load 102 depends on the height of the load, where more misalignment is found with taller loads. For example, when a 90 inch high load with a center of rotation 140 of the turntable 122 is 1 1/16 of an inch misaligned from the rotational axis 149 of the platen paid 105, the rotational axis 149 of the platen pad 105 and the center of rotation 140 of the top of the load 102 may be misaligned by as much as ⅝ inch. Similarly 1° of misalignment at the bottom of the load 102 may create a misalignment of 3 or 4 inches at the top of the load 102. A misalignment of ½ inch is enough to create unwanted grinding between the load 102 and the platen pad 105.
Conventionally, alignment techniques such as the use of a plumb bob may permit alignment of rotational axis 149 of the platen pad 105 and the center of rotation 140 of a load 102 of a given height. However, this and other similar alignment techniques are cumbersome and take time. The alignment of the rotational axis 149 of platen pad 105 and center of rotation 140 of load 102 must be recalibrated each time a load of a different height is used. Even when the same height of load is used, periodic recalibration is required due mechanical wear and flexing of machine parts.